1. Field of the Invention
The present invention relates to measurement of impedance using an ordinary digital multimeter which was originally designed to provide only DC test voltage. Specifically, an AC resistance converter enhances the capabilities of conventional digital multimeters by providing both the ability to measure impedance of active components (capacitors and inductors) and by also providing more accurate measurements for tests for which DC test voltages/currents obscure results or effect the material being tested.
2. Description of the Related Art
It is estimated that at least 10 to 20 million digital multimeters (DMM) are in use in the U.S. Conventional DMMs are designed to measure five basic functions: (1) DC voltage (2) DC current (3) DC resistance (4) AC current and (5) AC voltage. But conventional DMMs are not designed to measure impedance, which reflects the capacitive, inductive and resistive components of an active circuit.
The conventional DMMs measure resistance using only DC test voltages. DC measurements do provide (1) high accuracy (2) reduced AC interference by using a DC to few hertz bandwidth (3) rejection of capacitive and inductive circuit components (4) low cost and (5) interchangeability.
Despite the conventional DMM's versatility, there are desirable functions that it cannot perform. Conventional DMMs are restricted to applications where the uni-directional test current does not alter the measurement's accuracy or obscure the data to be measured by the DMM. If there is a difference in the ohmic bipolar measurement caused by the presence of unbalanced currents, non-linear conduction, etc., the DC test current of conventional DMMs is ineffective.
The DMM manufacturers frequently include additional features in the DMMs, such as continuity, frequency, capacitor, diode and transistor tests, and conductivity functions. Ironically, many of the conductivity measurements must be made by AC and not by DC signals to preserve true polarization and ion balance in the materials being tested.
There are a wide variety of potential applications for a DMM that provides AC test voltages. For example:
AC resistive and reactive components can be measured with vector quantities in the RLC series mode. The phase angle is known once the capacitance is measured. Resonance effects can be studied; Resistances can be measured up to 10.sup.10 ohms.
Such a device could be used to measure non-voltage producing non-linear elements. Non-linear elements, materials, fluids, and gases do not produce a proportional change in current with the applied voltage in one or bipolar direction. For example, a diode is a non-linear unipolar element allowing current to flow in one direction. Another example is the tightly-bound conducting molecular polymer chains containing positive and negative polarized ions. Ohm measurements using DC voltage will affect the ion balance and true information may be lost.
Such a device could be used to measure the AC resistance of DC devices and sources. The internal AC resistance of low current watch batteries when new may be 5 to 10 ohms. Their internal resistance increases with use. DC battery testers fall short of the AC tests as they use an arbitrary DC test current to measure a non-linear resistance.
Insulation resistance tests of cables and materials are conducted with DC voltages. Due to aging and corona discharge (ionization of the air), the quality of the dielectric degrades. Insulation deteriorations can be measured with an AC capacitive-coupled impedance meter or a conventional DMM with an R/Z converter.
The R/Z converter can also be utilized to measure conductivity. The passage of electrons or ionized atoms in fluids causes conductivity. To avoid chemical reactions caused by DC DMM's test probes in fluids which obscure the effective resistance, conductivity measurements are made by AC voltages. A conductivity probe or a noble metal interdigitized ceramic sensor may be used. Conductivity variations between de-ionized distilled or tap waters exceed 100 to 1 ratio. Water conductivity tests are widely used to verify safe acceptable limits.
Applications are also possible for voltage producing processes. For example, under normal use, diodes do not produce voltage but when the p-n junction is excited to higher energy levels by intense light or radioactive radiation, it emits electron flow and produces several tenths of a volt open-circuit potential. Similarly, in chemical processes in oxidation (a chemical reaction in which a compound loses electrons) or in reduction (when the compound gains electrons), several volts may be generated between electrodes or electrode-like surfaces.
Membrane potentials provide more important applications. A difference in potential that exists when two dissimilar electrodes are connected through a solution or gel causes electrochemical reactions. This is also referred to as membrane potential as a potential difference exists through the living cell membranes. These voltages are also present in patient-lead connected electrodes. When the skin surface of the patient and electrodes are connected through a conductive means, such as, gel or when taking electrocardiograms (ECG) or electroencephalogram tests, typical DC levels are from 0 to +150 millivolts and may reach 500 millivolts. These voltages can cause interference when taking ECG readings. To avoid errors caused by the DC offset, AC impedance measurements are used for ECG tests or when testing impedances of patient-connected electrodes.
Galvanic reactions and corrosions occur when dissimilar materials (metals, salts, etc.) when conduction exits between them. Conductive paths can be through humid air, gases, or fluids. The oxidation rate of metals or alloys can be measured and neutralized.
The Peltier Effect occurs when different types of metals are joined together at different temperatures, producing a DC voltage differential, unbalancing the DC resistance measurements. This principle is used in cooling, due to current flow, heat is generated and absorbed at the junction of the two dissimilar metals.
Between the space of any two electrically-connected material, a DC voltage is generated. To avoid errors at low resistance measurements, an AC test signal is often used.